It is typical for many phosphate ores to contain fluoride which is not readily separated from the phosphate but needs to be eliminated to at least some extent dependent on the use to which will be product dicalcium phosphate or monocalcium phosphate is to be put. As a general rule, dicalcium phosphate for use in animal feeds must, for example, contain less than 1000 ppm (parts per million) fluoride.
One procedure typically used is to treat the ore with sulphuric acid which produces a solution of phosphoric acid and hydrogen fluoride and a residue containing other contaminants and calcium sulphate. The solution is treated with calcium carbonate to a pH of about 2.9 to 3.0 in order to precipitate calcium fluoride and thereafter hydrolyzed to yield a precipitate of calcium fluoride and produce a solution of calcium and phosphate ions which can be directly processed to form a usable dicalcium phosphate product. However, in carrying out this process, an amount of dicalcium phosphate is precipitated together with the calcium fluoride and this combined precipitate is discarded as a waste product. The quantity of phosphate discarded in this way can be from 10 to 20 percent of the phosphate present and thus represents a significant loss.